Designing a sheepdog to herd photons

Researchers develop a tool that lets them measure how the environment tells a …

One of the nice things about electronic devices is that it is really easy to guide electrons. Even if you don't use wires, you can use a combination of electric and magnetic fields to control where electrons go. We can do this right down to the level of single electrons and really small distances. Light, however, is a different story.

Sure, we have optical fibers, optical elements like lenses and mirrors, and, of course, lasers. To an extent, these tools give us the sort of control that we have over electrons. But this doesn't extend down to the single photon level or to very small optic hardware like photonic devices. The analogy would be that we have the optical equivalent of transmission lines and switching stations, but we don't have an optical radio or integrated circuit—no, the current generation of integrated optics doesn't count.

Control at the single photon level requires some particularly careful engineering. Rising to the challenge is a group of researchers at the Institute for Atomic and Molecular Physics (AMOLF) in the Netherlands who have developed a tool that lets them precisely control how light gets into a device.

Where do all the photons go?

To understand why this particular bit of research is interesting, we need to understand why we can't control light like we control electrons. Imagine that I have a dye molecule sitting in space. I give it a good kicking, putting it into an excited state. It is now ready to emit a photon. But what color photon? What direction will it go? What polarization will it have?

The answers to these questions are given in probabilities, not certainties. For instance, when the dye molecule emits a photon, it does so because an electron makes a transition between an excited state and a ground state. But the energy difference between these two is a mean value, and it is possible that the transition that takes place has considerably less energy than the mean.

In fact, there is a relationship between how long an electron spends, on average, in the excited state and the range of energies it will emit during its transition back to the ground state. The longer it stays in the excited state, the narrower the energy range. So, it is possible to have certainty about a photon's energy—provided you are willing to wait forever to see a photon.

The direction and polarization of the photon depend on the orientation of the dye molecule and the direction from which we observe it. To emit the photon, the electron has to oscillate rapidly. Now, let's imagine that this oscillation is a circular motion. If we happen to be looking down on the electron as it orbits, then we will only ever see it emit circularly polarized light. If we happen to be looking from the side, then we will only ever see light that is linearly polarized, with the orientation determined by the orientation of the dye molecule.

The chances of detecting a photon is different for an observer at the top versus an observer at the side.

You're going to bring up modes, aren't you?

Excited dye molecules have all of these possibilities available to them because of their own state. But there is one more factor governing where the photon will go: the environment. Let's take a simple example. We put our dye molecule near a silver mirror. When the excited electron starts to oscillate, electrons in the mirror start to oscillate as well. But their oscillations mirror the oscillation of the electron in the dye molecule.

Combined, the mirror and the electron are going to emit a single photon. But now it can't just go anywhere; the two oscillations must add up in phase. If the molecule and the mirror are very close to each other—a distance much smaller than the wavelength of the light they will emit—then they can't emit a photon directly away from the mirror, because the oscillations are out of phase with each other. However, they can emit parallel to the surface of the mirror. If the spacing is exactly half a wavelength, then emission directly away from the mirror is enhanced while emission parallel to the mirror is suppressed.

If you remember, I said that there is some uncertainty in the emission energy (which is related to the wavelength). Well, now the chances that the dye molecule will emit a photon that has a wavelength that is half that of the distance between dye molecule and the mirror is approximately doubled.

All of this occurs because they dye molecule must emit into something called a mode, and the available modes are determined by the environment. So our mirror automatically blocks some modes through interference effects and enhances others by effectively replicating them. In doing so, it changes the probabilities of where photons go, when they are emitted, and what color they have.

That is how you control single photons: make sure that the mode you want is greatly enhanced with respect to the others. That's easy to do in principle, but it requires fabricating structures with reasonably high precision—which we can do thanks to the electronics industry—and placing photon emitters with nanometer precision with respect to the structures.

The latter is something we can't do so easily for two reasons. The big problem is that the production techniques used to make single photon emitters are not usually the same as those used to make photonic structures. The emitters have to be added after the fact, and they often end up in the wrong location. The second problem is that we can't always predict exactly where an emitter should be placed in practice. That means that, even if we could place emitters where we wanted during production, we would likely get it wrong.

Does this mean you are getting to the point?

The researchers from AMOLF have taken a very sharp tip and stuck dye molecules embedded in a polystyrene ball on the end. In their setup, this tip is held in place within a few nanometers of the sample while the sample—in this case, the sample was a glass plate with a gold nanowire on top—is raster scanned under it (think slid past line-by-line). Whenever the dye molecules move into an area where it is favorable to emit into the wire, it does so rapidly.

What the researchers observe is that the lifetime (that is, the time it takes for half the excited dye molecules to emit) gets much shorter when they preferentially emit into the wire. This allowed them to show where an emitter should be placed to ensure that as many photons as possible are emitted in the intended direction.

Why is this important? Simply put, it allows researchers to make direct comparisons between how light is supposed to propagate through photonic structures and how it actually propagates. This can then be used to rapidly and iteratively improve structures.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.